Shape memory alloy with ductility and a making process of the same

ABSTRACT

A Ti—Ni shape memory alloy with ductility, including Ti of 50˜66 atomic % in a composition, and in which precipitation of Ti 2 Ni phases at grain boundaries is suppressed.

FIELD OF THE INVENTION

The present invention relates to a Ti—Ni shape memory alloy and a making process of the same. More particularly, the present invention relates to a new Ti—Ni shape memory alloy which is excellent in mechanical properties with which the Ti—Ni shape memory alloy can be put to practical use as an actuator for micro machines such as a micro valve. The present invention also relates to a making process of the Ti—Ni shape memory alloy.

DESCRIPTION OF THE PRIOR ART

A Ti—Ni alloy has been known as a shape memory alloy. A making process in which the Ti—Ni shape memory alloy is produced as a thin film has also been known. Besides, it has been known that a Ti—Ni alloy with excessive Ti, the amount of which is 50˜66 atomic % in a composition, has higher R-phase transformation temperature than those of Ti—Ni alloys either with excessive Ni in a composition or with a composition in which Ti and Ni have an equal atomic ratio.

While these Ti—Ni alloys with lower transformation temperature has been put to practical use, the Ti—Ni alloy with excessive Ti has so poor mechanical properties and, is so brittle that, in fact, it has not been used so far.

The present invention has an object to overcome the defect of the Ti—Ni alloys above-mentioned and to provide a new Ti—Ni shape memory alloy with excessive Ti, which can be actuated at room temperature and has enough mechanical properties to be put to practical use.

This and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed specification and drawing, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photo showing a structure of a Ti—Ni alloy thin film obtained by annealing at 773K for 5 minutes;

FIG. 2 a is a photo showing a structure of a Ti—Ni alloy thin film obtained by annealing at 873K for 3 minutes;

FIG. 2 b is a photo showing a structure of a Ti—Ni alloy thin film obtained by annealing at 873K for an hour;

FIG. 3 illustrates several stress-strain curves of a Ti—Ni alloy thin film with the structure as shown in FIG. 2 b;

FIG. 4 illustrates several stress-strain curves of a Ti—Ni alloy thin film with the structure as shown in FIG. 1;

FIG. 5 illustrates several stress-strain curves of Ti—Ni alloy thin films obtained by annealing at 873K for 3˜60 minutes;

FIG. 6 illustrates a diagram showing a relationship between plastic strain of Ti—Ni alloy thin films at room temperature and annealing temperatures;

FIG. 7 illustrates the result of a heat cycle test relating to the Ti—Ni alloy thin film as shown in FIG. 1; and

FIG. 8 is a photo showing a structure observed in a usual ingot material.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a Ti—Ni shape memory alloy with ductility, which includes Ti of 50˜66 atomic % in a composition and in which precipitation of Ti₂Ni phases at grain boundaries is suppressed. More particularly, the present invention provides a Ti—Ni shape memory alloy with ductility, which exhibits plastic strain of not less than 15% at room temperature.

The present invention further provides a making process of a Ti—Ni shape memory alloy with ductility, which includes Ti of 50˜66 atomic % in a composition. The process comprises the steps of crystallizing an amorphous Ti—Ni alloy at 600˜900K for less than an hour and then cooling down to room temperature slowly in order to suppress precipitation of Ti₂Ni phases at grain boundaries. More particularly, the present invention provides a making process in which the crystallization is conducted at 800˜900° C. for less than 10 minutes.

Suppressing precipitation of Ti₂Ni phases at grain boundaries remarkably improves mechanical properties of a Ti—Ni alloy with excessive Ti.

Any special condition is not required for alloy composition except that the amount of Ti is within the range of 50˜65 atomic %. The alloy of the present invention mainly consists of two metallic elements, i.e., Ti (titanium) and Ni (nickel), but as far as both Ti₂Ni and TiNi phases are precipitated in a usual ingot material, other elements than Ti and Ni may be added or mixed as an impurity within the range where crystalline structures peculiar to those two phases are preserved.

On the other hand, in the case that the amount of Ti is less than 50 atomic %, Ti₂Ni phases are not precipitated in a usual ingot material. In the case of Ti of beyond 66 atomic %, TiNi phases are not precipitated and, as a result, effects of the present invention are not exhibited.

Once a usual ingot material is subjected to high temperature, Ti₂Ni phases are preferentially precipitated at grain boundaries which are stable in energy and this precipitation severely deteriorates mechanical properties of the material. For example, FIG. 8 illustrates a structure observed in a usual ingot material (H. C. Lin, Shyi-kaan Wu, and J. C. Lin: Proc. ICOMAT'92, J. Perkins ed., Monterey Institute of Advanced Studies, Monterey, Calif., 1992, pp. 875-880). In FIG. 8, an estimated scale is inserted into for reference. Precipitation of Ti₂Ni phases at grain boundaries is confirmed.

In the present invention, in order to prevent Ti atoms from dispersing to grain boundaries and make an alloy in which precipitation of Ti₂Ni is suppressed, an amorphous alloy is crystallized in a temperature range of 600˜900K for a short time, i.e., less than an hour.

Crystallization time is varied according to the size and the shape of an alloy, but a short time is preferable. Time suitable for crystallization is within the middle of minutes of two figures. Ten minutes or shorter are considered to be more suitable.

A typical annealing condition is exemplified as 773K for 5 minutes. Preferably, a condition of 800˜900K for less than 10 minutes is exemplified. In the latter condition, plastic strain of not less than 15% is obtained at room temperature, this realizing a Ti—Ni shape memory alloy which is not fractured in working and practical use.

An amorphous Ti—Ni alloy is produced as a thin film by a vapor deposition or any arbitrary technology. A making manner of it is not restricted.

The alloy of the present invention as a thin film is expected to be applied to an actuator for a micro machine and its importance is emphasized.

Now, an alloy and its making process of the present invention will be described more in detail by way of examples, but it is needless to mention that the present invention is not restricted to these examples.

EXAMPLES

Using a target of a Ti—Ni alloy, an amorphous thin film consisting of a Ti-48.3 atomic % Ni alloy with the thickness of about 7 μm was formed on a glass substrate. The alloy thin film was annealed within a temperature range of 600˜900K and then cooled down to room temperature slowly. A structure of the alloy thin film after annealing was observed by an electron microscope. FIG. 1 and FIGS. 2 a and 2 b are photos of a typical structure. In the alloy thin film subjected to annealing at 773K for 5 minutes, as shown in FIG. 1, a slight amount of oxides are confirmed at grain boundaries, but Ti₂Ni phases are scarcely precipitated. The same structure as this is confirmed in the alloy thin film annealed at 873K for 3 minutes, as shown in FIG. 2 a. On the contrary, in an alloy thin film subjected to annealing at 873K for an hour, as shown in FIG. 2 b, precipitate of wedged-shaped Ti₂Ni phases is observed at grain boundaries, especially at triple points of grain boundaries.

The stress-strain curves of the Ti—Ni alloy thin film annealed at 873K for an hour, as shown in FIG. 3, are the results of measurement at several testing temperatures. Even at each temperature, strain relating to martensitic transformation is confirmed, but the alloy thin film was fractured during elastic deformation of martensite phases. Plastic deformation is not confirmed. Plastic strain is zero and no ductility exhibits at the time of the elastic deformation.

On the contrary, it is confirmed from several stress-strain curves as shown in FIG. 4 that the Ti—Ni alloy thin film annealed at 773K for five minutes exhibits plastic deformation as high as 5˜12% after martensite phases is yielded. It is estimated from this fact that the alloy thin film has enough ductility to be put to practical use. The ductility of the alloy thin film is such a property as has never been obtained in conventional ingot materials.

The stress-strain curves as shown in FIG. 5 are the results of measurement at room temperature, i.e. 20° C. Specimens were Ti—Ni alloy thin films which were annealed at 873K for 3˜60 minutes. Plastic strain deteriorates with the length of annealing time. The Ti—Ni alloy thin film annealed at 873K for 3 minutes exhibits plastic strain of about 25%, which is the best of all specimens.

The diagram as shown in FIG. 6 shows a relationship between plastic strain of Ti—Ni alloy thin films at room temperature and annealing temperatures. Annealing time of these alloy thin films was 5 minutes and was common to all specimens. Plastic strain of not less than 15% is obtained at room temperature in the case that the annealing condition is 800˜900K for less than 10 minutes. The ductility realizes a Ti—Ni shape memory alloy which is not fractured in working and practical use.

From FIG. 7, which illustrates a shape memory property of the alloy thin film annealed at 773K for 5 minutes, it is confirmed that high transformation temperature which is one of the characteristics of the alloy with such a composition is maintained and that a complete shape memory effect exhibits at not less than room temperature. It is particularly important that the shape memory effect as shown in FIG. 7 is not resulted from martensitic transformation observed in a usual ingot material, but from R phase transformation. Since the strain of R phase transformation is smaller than that of martensitic transformation, the amount of plastic deformation which is introduced at the time of transformation is small enough. This fact as well as improvement of mechanical properties above-mentioned is thought to be useful for improving reliability of use for a long time. Besides, since temperature hysteresis is small, an actuator with high response can be possibly realized. 

1. A Ti—Ni shape memory alloy composition comprising Ti in an amount of 50-66 atomic % of said composition, and wherein the Ti—Ni shape memory alloy composition is ductile and wherein precipitation of Ti₂Ni phases at grain boundaries is suppressed.
 2. The Ti—Ni shape memory alloy composition as claimed in claim 1, wherein the Ti—Ni shape memory alloy composition exhibits plastic strain of not less than 15% at room temperature.
 3. The Ti—Ni shape memory alloy composition as claimed in claim 1, wherein the Ti—Ni shape memory alloy composition is produced as a thin film.
 4. The Ti—Ni shape memory alloy composition as claimed in claim 2, wherein the Ti—Ni shape memory alloy composition is produced as a thin film.
 5. A process of making the Ti—Ni shape memory alloy composition as claimed in claim 1, which process comprises crystallizing an amorphous Ti—Ni alloy at 600-900K for less than an hour and then cooling down to room temperature slowly in order to suppress precipitation of Ti₂Ni phases at grain boundaries.
 6. The process as claimed in claim 5, wherein crystallization is conducted at 800-900K for less than 10 minutes.
 7. The process as claimed in claim 5, wherein the Ti—Ni shape memory alloy composition is produced as a thin film.
 8. The process as claimed in claim 6, wherein the Ti—Ni shape memory alloy composition is produced as a thin film.
 9. The Ti—Ni shape memory alloy composition as claimed in claim 1, which comprises Ti and Ni.
 10. The Ti—Ni shape memory alloy composition as claimed in claim 9, wherein one or more elements other than Ti and Ni is added or mixed as an impurity.
 11. The Ti—Ni shape memory alloy composition as claimed in claim 10, wherein said element(s) is/are in an amount which preserves crystalline structures of Ti₂Ni and TiNi phases. 